BARTOW, Fla. — A small plane headed to Key West for the Christmas holiday crashed Sunday at the end of a runway at a Florida airport, killing five people, authorities said.

The twin-engine Cessna 340 crashed shortly after takeoff at Bartow Municipal Airport, immediately killing the 70-year-old pilot and his four adult passengers, according to a statement from Polk County Sheriff’s Office spokeswoman Carrie Horstman. It wasn’t immediately clear what caused the crash.

The sheriff’s office identified the pilot as Lakeland attorney John Shannon. He had filed a flight plan to fly Sunday from Bartow to Key West, Horstman said.

Shannon’s passengers included his two daughters, his son-in-law and a family friend. They were identified as: 24-year-old Southeastern University student Olivia Shannon, 26-year-old Baltimore teacher Victoria Shannon Worthington, 27-year-old University of Maryland law student Peter Worthington Jr. and 32-year-old Lakeland teacher Krista Clayton.

The Worthingtons had arrived in Florida on Saturday for the Christmas holiday, Horstman said.

“This is a tragedy at any time, but it is so much worse because it happened on Christmas Eve,” Sheriff Grady Judd said.

The plane was engulfed in flames by the time fire rescue crews arrived, Polk County Fire Rescue spokeswoman Tina Mann said in a statement.

The private plane was heading east into heavy fog when it took off, according to the sheriff’s office.

The National Transportation Safety Board and the Federal Aviation Administration will be investigating the crash.